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Avalanche Forecast

Archived

Feb 24th, 2021–Feb 25th, 2021

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

South Columbia.

Watch for fresh and reactive storm slabs forming Thursday as new snow arrives amid strong wind. Keep in mind the potential for triggering deeper slabs may persist even as natural activity from the previous storm tapers off.

Confidence

High -

Weather Forecast

WEDNESDAY NIGHT: Increasing cloud with flurries bringing up to 5 cm, light wind increasing to strong southwest, freezing level valley bottom.

THURSDAY: 5-10 cm of new snow, strong southwest wind easing to moderate, freezing level 1200 m.

FRIDAY: 5-15 cm of new snow, light to moderate northwest wind, freezing level 1000 m.

SATURDAY: A trace of new snow, light to moderate southwest wind, freezing level 1000 m.

Avalanche Summary

During the storm last weekend and into Monday, the northern Selkirks were host to a cycle of very large natural and explosive triggered avalanches size 2-4, while the Monashees had surprisingly limited activity up to size 2. The storm was late to the southern half of the region which was quiet until reports from Monday described natural, explosive and skier triggered avalanches size 1-2, as well as one report of a natural cornice triggered size 3.

Post-storm in the Monashees, slabs quickly became stubborn to trigger, as described in these MIN reports from Monday here and here. On Tuesday in the Selkirks near Revelstoke, a skier remote size 2 was reported while explosive work just south of town produced limited results including a few size 2 storm slabs on S-SE aspects in the alpine.

We have had few reports from the south of the region of the odd persistent slab avalanche running on the late January interface. These have been around size 2, skier triggered out of steep, cliffy, unsupported terrain around treeline. At least one occasion was in a location where an avalanche had previously slid and the bed surface reloaded.

Snowpack Summary

10-20 cm of new snow is forecast to fall by the end of the day Thursday. It falls over 40-80 cm of recent storm snow which is well settled at lower elevations and heavily wind affected at upper elevations. Wind deposits up to 100 cm deep can be found in lee terrain features in the alpine and may remain reactive to human and machine triggers. 

All of this recent snow sits over facetted surfaces from the mid February cold snap. There is uncertainty around how long it will take for this interface to bond but reports from the Monashees have indicated a positive trend already.

We've now got 70 to 120 cm of snow above the late January persistent weak layer which is composed of surface hoar in openings in the trees and/ or a crust on steep solar aspects. Very isolated activity on this layer has been observed recently, usually skier triggered in steep, cliffy, unsupported terrain around treeline in the south of the region.

Terrain and Travel

  • Be carefull around freshly wind loaded features.
  • Fresh snow rests on a problematic persistent slab, don't let good riding lure you into complacency.
  • Keep in mind that human triggering potential persists as natural avalanching tapers off.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Persistent Slabs

Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.